The present invention relates to a deaeration vessel used in the paper and pulp industry. Especially, it relates to a deaeration vessel comprising the vessel itself, a partition wall arranged therein and forming a weir K1 and, arranged in the vessel jacket, inlets for a first medium to be introduced into the vessel, outlets for a substantially gas-free first medium, an outlet for separated gas, and an outlet for a surface flow/fraction of the first medium, which surface flow/fraction has been separated by weir K1.
Experts of the field know these deaeration vessels best by the trademark DECULATOR.RTM. (A. Ahlstrom Corporation). These deaeration vessels are used especially in the paper and pulp industry. As implied by their name, these vessels are purposed for removing gas, e.g., from a fiber suspension.
Deaeration vessels, like the one discussed in DE-A1-42 34 522, are large vessels, into which the fiber suspension to be fed to the papermaking machine is introduced as a dilute mixture. The inside of the deaeration vessel is normally divided into two compartments by a weir. The operating principle of the vessel is such that the dilute fiber suspension is introduced into the vessel at the first side of the weir where the suspension is subjected to sub-atmospheric pressure which separates the gas into a so-called gas space from where it is discharged from the vessel. A further operational feature of the vessel is to allow other light impurities to be separated onto the liquid surface in the vessel because of their lightness and then discharge them along with the surface flow/fraction from the vessel to the second side of the weir. The suspension directed to the papermaking machine is withdrawn through an opening in the bottom of the vessel at the first side of the weir, the idea naturally being to withdraw fiber suspension in as gas-free a state as possible from the vessel. In order to maximize the deaeration capability of the vessel, the vessel is evacuated by means of a vacuum pump, whereby the gas is withdrawn from the vessel via the vacuum pump. The deaeration capability is further enhanced by introducing liquid via nozzles above the level of liquid in the vessel at the first side of the weir and by causing the introduced liquid to hit, e.g., the ceiling of the deaeration vessel, whereby the gas, if any, present in the liquid is the form of bubbles is separated prior to the liquid becoming into contact with the liquid in the vessel. Equipment in accordance with a so-called DECULATOR.RTM. Flying Wing process disclosed in, e.g., Finnish patent 47795 (U.S. Pat. No. 3,538,680) is used for cleaning of papermaking stock and removing of air/gas therefrom.
It is previously known that the final consistency of the stock to be fed from the headbox to the wire section is preferably fine-adjusted just before the headbox or in connection therewith. This fine adjustment requires deaerated fresh water or equivalent.
The medium being introduced into the papermaking machine and substantially comprising stock is hereinafter referred to as a first medium. The white water, fresh water, or equivalent needed for the fine adjustment of the stock consistency is hereinafter referred to as a second medium. The second medium is deaerated in a separate deaeration means, in accordance with prior art.
Our invention relates to an improved apparatus for preparing the gas-free or substantially gas-free second medium, required for the fine-adjustment of the stock consistency.
It is a characteristic feature of the deaeration vessel according to the invention that the same gas space where the first medium is treated in the vessel, is provided with a second partition wall forming a second weir K2 and that the vessel jacket is provided with an inlet/inlets for a second medium to be introduced into the vessel and with an outlet for a substantially gas-free second medium, and that the common gas space is provided with an outlet known per se, which outlet is intended for the gas separated from both the first and the second medium, and with an outlet for the surface flow/fraction of the second medium separated by weir K2.
In accordance with the arrangement of the invention, the deaeration of the second medium employs the same equipment as the deaeration of stock to be introduced into the headbox. Thus the water, preferably white water, used for adjusting the final stock consistency is preferably taken from the wire pit and conveyed to a deaeration vessel, which may be of, e.g., the same type as DECULATOR.RTM. deaeration vessels and in which the water is deaerated. The equipment in accordance with the invention is easy to connect with prior art deaeration vessels. For example an end of a cylindrical deaeration vessel is readily provided with means, such as necessary conduits and weir, needed for deaeration of dilution water, i.e., the second medium, by extending the vessel. The arrangement of the present invention utilizes means existing in the deaeration vessel, for removal of gas. Deaeration of the first and the second medium in one and the same gas space, i.e., in one vessel, brings an obvious space saving. The equipment also forms a whole. If the so-called Flying Wing type deaeration means is used, it is easy to add deaeration means for the second medium in accordance with our invention to an end of this structure.
The surface flow/fraction of the second medium, separated by a weir, may be discharged from the vessel through a separate outlet. In that case, weirs K1 and K2 have a partition wall between them, to separate the surface flows/fractions, separated by a weir, of the first and the second medium from each other.
When the second medium has been deaerated, this second medium, which is e.g. water removed from the wire of the papermaking machine, fresh water, or a mixture of these, is conveyed via a pump and appropriate screens to the headbox, where the ultimate fine-adjustment of the consistency of the stock to be fed to the wire is effected. By white water is meant water discharged from the wire of the papermaking machine.